1. Field of the Invention
The invention relates to optical sensors and, more particularly, to an optical liquid sensor.
2. Background Information
Liquid sensing capability is important to the safe operation of portable UV water treatment devices. By sensing when the UV light source is and is not immersed in water, light source operation may be either enabled or disabled to ensure safety. Allowing a UV light source to turn on only when it is fully immersed in water protects the user against is potentially dangerous UV exposure.
In addition to UV light source control, there are many other applications in which liquid sensing is important. These include water and fuel level sensing in storage tanks, water level sensing in boat bilges and buildings (basements) for pump and alarm activation, water sensing in watertight compartments for alarm activation, and water sensing for automatically operating lifejackets, life raft or lifejacket emergency lighting, marine emergency radio beacon activation (EPIRB), etc.
Liquid sensing is often accomplished by measuring electrical conductivity using two electrodes, normally at a relatively low voltage. When the electrodes are held in air, they encounter an extremely high resistance and virtually no current flow occurs. When the electrodes are in liquid, they encounter a lower resistance than that of air and some small but measurable current begins to flow. By measuring this current or its associated voltage, control circuitry can determine the presence or absence of liquid and trigger actions such as enabling/disabling UV light source operation.
While conductivity sensing can be fairly reliable, it does have some weaknesses. For example, if the liquid in which the electrodes are immersed is of a very low conductivity, as in the cases of distilled water or snowmelt water, the current flow between immersed electrodes may be extremely low (in the nanoamp range) and difficult to use for reliable sensing. In response, the sensitivity of the liquid sensor circuitry must be increased by amplifying the very weak current between the electrodes, so that immersion in the low conductivity liquids may be sensed. This higher sensitivity, however, may result in other problems. In particular, sensors set to a very high sensitivity may detect latent moisture on the electrodes as immersion. In addition, high conductivity may cause associated control electronics to function in a manner that is not as intended. For example, the sensor operation may result in the UV light source continuing to operate after the light source is removed from the water due to the latent is moisture on the electrodes being sensed as continued immersion.
In addition to the problems associated with sensing low conductivity liquids, there are corrosion issues associated with conductivity based liquid sensors. When current flows between the sensor electrodes, electrolysis occurs and, over time, the electrode surfaces corrode. The corrosion may cause a change in the mechanical and electrical characteristics and this, in turn, may cause problems with sensor function. Over long periods, electrolysis may even destroy the sensor electrodes, and thus, cause system failure.
Another method of liquid sensing uses light rather than conductivity. This optical sensing approach takes advantage of the differences in refractive indexes of air and water. Normally the optical sensor consists of a light source such as a light-emitting diode (LED) device, a light-sensing component or photo sensor, such as a photodiode or a phototransistor, and a precise conical lens or a prism generally in the form of a cylindrical quartz rod, that is, a lens or prism manufactured to exhibit calculated characteristics. Typically, the light source and photo sensor are precisely positioned side by side under the conical lens, such that the lens allows the light from the light source to pass through when the lens is in water and causes the light emitted from the LED device to be reflected in calculated paths and directions toward the photo sensor when the lens is in air. The prisms operate in essentially the same manner when the light source and the photo sensor are precisely positioned relative to the prism. Sensors using a prism or conical lens tend to be relatively costly, and thus unprofitable, for use in lower cost products, e.g., the SteriPEN® UV Water Purifier from Hydro Photon.
In addition to being costly, the optical sensors incorporating the conical lenses and prisms may malfunction in environments in which ambient light is relatively bright. In such environments, the ambient light enters the conical lens and is also directed in calculated paths and directions toward at the photo sensor. Thus, such a sensor in a brightly lit water tank may not trigger as it should, i.e., as the water level changes, because the photo sensor cannot distinguish between the ambient light and the light produced by the light source in the light that is directed by the conical lens or prism to the photo sensor.